Plant, generator and propeller element for generating energy from watercurrents

ABSTRACT

A plant, generator and rotating member for the production of power from currents in a body of water, comprising a fixedly mounted of floating structure, and a plurality of replaceable generator units ( 750 ) supported by the structure and which are driven by the water currents. The structure comprises arms ( 615, 720, 730 ). The rotating member ( 400 ) comprises a plurality of member sections ( 410 ) rotatably mounted on a shaft ( 405 ) between an end piece ( 407 ) and a tip ( 406 ). The generator comprises a contra-rotating rotor ( 550 ) and stator ( 800 ) connected to respective shafts ( 500, 820 ) and bearings, where the stator frame ( 800 ) is axially supported ( 810 ) on the first shaft ( 500 ) and the first shaft ( 500 ) at one end thereof is axially supported ( 810 ) on the stator frame. The generator according to the invention can be used for the production of electric power, and as an electromotor for the production of mechanical rotational energy.

TECHNICAL FIELD

The invention relates to apparatus for extracting energy from watercurrents. More specifically, the invention relates to plants for theproduction of energy from currents in a body of water, comprising astructure capable of resting on a bottom below the body of water or offloating on the body of water and a plurality of replaceable generatorunits supported by the structure and which are driven by the watercurrents, and a rotating member for use as energy supplier in agenerator unit located in a body of water a generator, more specificallya generator in which the stator and the rotor are rotatable relative toone another. The plant utilises currents below the surface of oceans,seas, rivers or other bodies of water. Such currents may, e.g., becaused by tidal variations and/or underwater topography (e.g., channels,riverbeds or other narrow passages under water).

The generator according to the invention can produce electric power byutilising any source of energy, but is especially suitable for beingdriven by wind and water currents, in particular low-speed watercurrents. The generator can also be operated as an electromotor.

BACKGROUND ART

Current in large bodies of water, as for instance that generated bytidal variations, is a renewable source of energy which thus far has notbeen exploited in Norway. This is the case even though such currents arehighly predictable and readily available per se along the Norwegiancoast.

When currents in large bodies of water (e.g., oceans or rivers) areforced through channels or other narrow passages, the velocity of flowwill increase and will to a very great extent be aligned. The current inthe central region of the narrow passage will have an almost equalvelocity across the whole of the cross-section in question, which meansthat it is advantageous to position a power generator in this region.

There are a number of known apparatus and methods for the production ofenergy from ocean currents.

Norwegian Patent Application 1999 1984 (Hammerfest Strøm) describes aplant for the production of electric power from ocean and rivercurrents. The whole of the plant is located below the surface of thewater and comprises a plurality of turbines having blades, a supportsystem, a system of stays and a generator. The turbine shafts areoriented perpendicular to the direction of movement of the water, andthe blades are wing-shaped so that the turbine rotates in the samedirection, regardless of the direction of movement of the water. Theturbine shafts are supported in a framework of buoyancy tanks, securedto the support and bearing system. The plant is built up of modules. Theplant has positive buoyancy regulated by the buoyancy tanks and a systemof stays secured below the surface of the water, so that the plant isheld below the surface of the water by the system of stays. The plantemploys conventional blades.

Danish Patent 155454 (Hans Marius Pedersen) describes a floating watercurrent power station which consists of a ring-shaped pontoon which bymeans of a bitt is anchored to anchors on the bottom. All the turbinesare replaceable and are arranged on a common beam and can as a unit beswung up to the surface within the area defined by the ring pontoon. Thepower station may move around the bitt, the upper end of which isconnected to a front pontoon and the lower end of which is secured tothe anchors.

U.S. Pat. No. 5,440,176 describes a submersible water turbine plantcomprising turbines/generators in different combinations suspended belowa submerged platform of the tension leg type.

Generators with contra-rotating rotor and stator are well known. In thefield of wind energy technology coaxially contra-rotating propellers areknown, inter alia, as a measure for extracting the rotational energywith which the air current is supplied when it passes the firstpropeller. The contra-rotating propellers may be on the same side of thegenerator, or on each side thereof. The propellers are either connectedby means of gears to the same generator, or one propeller is connectedto the rotor and the other propeller is connected to the field coils.The previously known contra-rotating propellers connected to the samegenerator require complex gearwheel drive and transmissions, whichresults in further energy loss. A contra-rotating propeller connected tothe rotor and the stator respectively is much simpler from a mechanicalpoint of view, but in the solutions known hitherto the field coilsrequire slip rings which may render the solution more complex, inaddition to there being an electrical and mechanical power loss.

In the known cases where the propellers are on each side of thegenerator (and the tower), it is preferable as a rule that thedownstream propeller should be smaller than the upstream propeller, andset to stall at a higher wind speed.

On the production of electricity from water currents, a lowest possiblepropeller speed is required. The consequence is a high torque and it isvery difficult to dimension a standard gear solution.

Known generator solutions usually have a constant rotational speed(pitch adjustment of the propeller) and stator that is stationary. Thisresults in the generator housing having a very large diameter, which isa disadvantage.

U.S. Pat. No. 4,291,233 describes a wind turbine generator havingoppositely rotating rotor and stator. Rotational energy from awind-driven turbine (propeller) having a preferably horizontal shaft istransformed into rotational energy in two preferably concentric shafts,via a bevel gear drive. The propeller shaft is attached to the bevelgear drive, which engages with upper and lower pinion gears that rotatein opposite directions. The pinion gears are secured to respectiveshafts which are preferably concentric and arranged vertically. The twoshafts—which rotate in opposite directions—are connected to a rotor anda stator respectively.

DE 43 04 577 A1 describes a wind turbine generator having two pairs ofvanes connected to a rotor and a stator respectively in order to turnthe pairs of vanes in opposite directions. Both pairs of vanes arearranged on the same side of the generator, and function as twocontra-rotating, double-bladed propellers. This document does notdisclose any further details with regard to the interaction of thestator and the rotor.

DE 196 43 362 describes a wind or turbine generator having oppositelyrotating rotor and stator. A shaft 8—having a first end 10 connected toa turbine or propeller 3—is secured to a rotor 9. A stator 13 is securedto a second shaft end 11, which in turn is connected to a turbine orpropeller 4. The shaft end 11 is supported on the shaft 8 via a bearing14, so that the rotor and the stator can rotate relative to each other.

Reference is also made to an auxiliary rotor 17 and an auxiliary stator18, where the rotor 17 is secured to the housing. Thus, the relativespeed between the components 17 and 18 is half of the speed between thecomponents 9 and 13.

The known contra-rotating generators can be divided in to two maingroups:

-   -   (i) the rotor and stator are driven by the same turbine or        propeller    -   (ii) the rotor and stator are supplied with rotational energy        from their respective turbine or propeller.

The first category of generator can be illustrated by U.S. Pat. No.4,291,233 (described above). The disadvantage of this generator is thatmuch energy is lost in the many transmissions that are used tocontra-rotate the stator. Furthermore, this generator is bulky, and willprobably be expensive both to manufacture and to maintain. Therefore,there is a need for a simplified type of generator, where the rotor andthe stator are driven by the same turbine or propeller, and where aminimum of gear transmissions are used and the use of bevel gears isavoided.

The second category of generator can be illustrated by DE 196 43 362(described above). The drawback with this generator is that it includesan outer member which does not rotate, and which is equipped with aconventional stator winding. This housing also has base lugs and takesup about half of the total torque.

The main cylinder is assumed to have magnets on both sides and isrotated by a separate shaft. Within this rotor there is another rotatingstator (or rotor). The torque that arises between the innermost rotor(9) and the magnet rotor (13) does not impact on the base lugs, but istaken up in the magnetic field between the propellers. Therefore, thereis a need for a simplified type of generator, where the rotor and statorare supplied with rotational energy from their respective turbine orpropeller, where the base is not required to take up the torque in thegenerator between the rotor and the stator, and where the torque of thepropellers is used in its entirety to generate electric power.

DISCLOSURE AND SUMMARY OF THE INVENTION

Thus, according to the invention there is provided a plant, a generatorand a propeller-type member for the production of energy from watercurrents of the type described above and as disclosed in the preamble ofthe attached patent claims.

The plant according to the invention comprises a structure capable ofresting on a bottom below said body of water, and a plurality ofreplaceable generator units supported by the structure and which aredriven by the water currents. The plant is characterised in that thestructure comprises a plurality of nodal elements having a respectivenodal centre and substantially horizontal arms projecting therefrom, andthat each nodal centre is made in the form of a bushing for aheight-adjustable leg extending towards the bottom.

The plant according to the invention thus comprises a structure capableof resting on a bottom (B) below said body of water, and a plurality ofreplaceable generator units supported by the structure and which aredriven by the water currents, wherein the plant is characterised in thatit can be positioned on the bottom below a body of water using saidsupporting legs which are movable through said bushings, and that theplant can selectively, with the aid of the supporting legs, be broughtto a chosen height in the body of water in order to optimise theposition of the generator in the body of water, or above the surface ofthe water so that said generator units are above the surface of thewater.

The plant according to the invention also comprises a floating plant forthe production of energy from currents in a body of water, comprising asupporting structure supported by a plurality of float members,characterised by a plurality of supporting arms which at one end thereofare pivotally attached to the structure, and at the other end thereofare attached to respective generators. In one embodiment the supportingarms comprise at least one joint between their first and second ends, sothat each supporting arm can be folded about the joint in order thereby,concurrently with the swinging of the supporting arm about the pivotalconnection of its first end with structure, to raise the respectivegenerator above the surface of the water.

The rotating member according to the invention comprises a shaft mountedon a hub on a shaft extending from a generator (not shown) to agenerator housing and intended for rotation actuated by currents in thebody of water, and characterised in that the rotating member comprises

-   -   an inner end piece mounted on said shaft, at the attachment of        the shaft to the hub;    -   a tip mounted at the outer end of said shaft;    -   a plurality of member sections mounted on said shaft between        said end piece and tip.

In one embodiment the member sections are rotatably mounted on andadjustable about said shaft.

In one embodiment the tip is rotatable and adjustable about said shaft.

According to the invention, there is also provided a generator of thetype described above and

Thus, the generator is characterised in that it comprises a transmissionmember fixedly connected to the shaft and which via transmission meansis attached to the stator, whereby—when the shaft rotates—the stator isrotatable in the opposite direction of the shaft.

In a second embodiment, the generator according to the invention ischaracterised in that the second shaft at one end thereof is connectedto a stator frame, axially supported on the first shaft; and the firstshaft at one end thereof is axially supported in said stator frame.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention are set forth in the attached patentclaims, and in the following description with reference to the attacheddrawings. It should be appreciated that the drawings merely illustratetypical exemplary embodiments and thus should not be regarded asdefining the limits of the invention. The same reference numerals areused to indicate the same parts in the drawings, and these are also usedin the description below.

FIG. 1 is a perspective view of a first embodiment of a tidal powerplant according to the invention.

FIG. 2 is a top view of the tidal power plant in claim 1.

FIGS. 3 a–3 c are top views of the tidal power plant in FIG. 2 and showtwo nodal centres (FIGS. 3 a and 3 c) and one intermediate piece (FIG. 3b).

FIG. 4 is a side view of the tidal power plant in FIG. 1.

FIG. 5 is a perspective view of one embodiment of a turbine unit androtating member, and shows a variant where the generator units areequipped with one rotating member rotatably arranged on each side ofsaid generator housing, and where the rotating members arecontra-rotating relative to one another.

FIGS. 6 a–6 c are perspective views of a second embodiment of the tidalpower plant according to the invention with the generators in a droppedposition.

FIG. 6 a is an end view of the plant;

FIG. 6 b is a side view of the plant; and

FIG. 6 c is a top view of the plant.

FIG. 6 d shows the plant in FIGS. 6 a–6 b seen from one end and with thegenerators in a retracted position.

FIG. 7 is a perspective view of a first embodiment of a rotating memberaccording to the invention.

FIG. 8 is (a) a simplified sectional diagram of a rotating membersection, and (b) a top view of a rotating member according to theinvention.

FIG. 9 is a perspective view of a second embodiment of the rotatingmember according to the invention.

FIG. 10 is a perspective view of a third embodiment of a rotating memberaccording to the invention.

FIG. 11 shows a first embodiment of the generator according to theinvention.

FIG. 12 shows a second embodiment of the generator according to theinvention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

In one embodiment, the plant according to the invention comprises astructure capable of resting on a bottom B below the surface S of a bodyof water. The structure supports a plurality of replaceable generatorunits 750 which are driven by the water currents. The plant structure isbuilt up of a plurality of nodal elements 700 having a respective nodalcentre, and substantially horizontal supporting members 720 andconnecting members 730 projecting therefrom. Upper and lower bushings711, 712 for a height adjustable leg 760 extending towards the bottom Bpass through each nodal centre.

The nodal elements 700 are interconnected by means of intermediateconnecting members 740 between the connecting members 730.

The generator units 750 are secured to substantially vertical supportingmembers 749, each of which may be secured to the horizontal supportingmembers 720 or the intermediate connecting members 740.

For production and hydrodynamic reasons, the arms, the supporting legs,the optional connecting members and the elongate supports are made inthe form of tubular members. Other cross-sectional shapes (e.g.,ellipse, rectangle) and configurations (e.g., latticework) should beregarded as being embraced by the present invention, even though suchforms and configurations are not considered to be ideal.

To facilitate the mobility of the plant during installation andsubsequent relocation, the supporting legs contain one or moreballastable chambers. At the same time, the supporting and connectingmembers constitute float members. When the supporting legs 760 aredeballasted, the plant is mobile. When the supporting legs 760 areballasted, the plant is stationary and can function in the currentconditions for the purpose for which it is designed without additionalanchoring.

When in operation, each individual generator unit should normally befixedly mounted to the supporting member, and the rotating member(s) ofthe generator should be capable of rotating both clockwise andanti-clockwise, depending upon the direction of the water current.However, in one embodiment the generator housing is pivotally supportedabout a vertical axis at the free end of the respective verticalsupporting member.

FIGS. 1 and 2 show a plant consisting of three nodal elements 700assembled by means of a corresponding number of connecting members 740.The plant thus describes an equilateral triangle with one supporting leg760 arranged in each corner. A configuration of this kind is favourableas the plant is statically determinant when it is ballasted towards thebottom.

To be able to cover as large a cross-sectional area as possible, thegenerator units are arranged so that the individual planes of rotationformed by the rotation of the rotating members 400 essentially overlapeach other. Thus, a minimum amount of water flows round the generators.

Although each generator unit will normally have two or more rotatingelements (see FIG. 1), it is also within the scope of the presentinvention to have generator units equipped with just one rotating member400, and these can also be arranged on each side of a generator housing(see FIG. 5). In this case, the rotating members are of coursecontra-rotating relative to each other.

A central aspect of the invention is that the plant can be positioned onthe bottom B via supporting legs 760. These are movable through bushings711, 712, so that the plant can selectively, by means of the supportinglegs, be brought into a chosen height in the body of water. A “jack-up”principle of this type is well known, but not in connection with plantssuch as those covered by the application. Because it is possible toadjust the height of the plant in the water, the generators can easilybe raised above the surface of the water for maintenance, repair orreplacement. Furthermore, the location of the generators in the body ofwater is optimised in relation to the current conditions and thetopography of the bottom. With the adjustable supporting legs accordingto the invention, the plant can thus easily be installed on a veryuneven bottom.

In another embodiment, the plant according to the invention comprisescontra-rotating turbines placed on a steel structure which floats in thewater, see FIGS. 6 a–6 d.

The plant comprises a platform 620 having a plurality of buoyancymembers 610, preferably one in each corner as can be seen in FIG. 6 c.The requirement of stability, both lateral and longitudinal, is thus metwith a minimum of displacement. A plurality of supporting arms 615extending out from the platform to each side each hold a generator 752and two turbines on its side, and are secured to a longitudinal pipewhich in turn is secured to all four pontoons. The supporting arms arehinged at their point of attachment and about halfway along. Operationof the supporting arms is effected using hydraulics.

The arrangement permits standard, known anchoring using anchor (notshown) and chain 617 in both directions: either one or two anchors ineach direction, or that one anchor in each direction holds twoplatforms. This means that the platforms can be anchored in thelongitudinal direction at a desired distance, and so close to each otherthat turbines run clear of each other. By anchoring several units sideby side, the lateral distance will also be adjustable or securable bymeans of chain.

After anchoring has been completed, chain cutters and anchor winchescould be removed and used on the next platform etc. During maintenanceand optionally when moving to a maintenance/repair facility (e.g., everythird year), the anchor winch is put back and chain is lowered to thebottom. On reinstallation, the chain is winched into the platform andreattached.

Production load from turbines is used to weigh the anchors in eachdirection. On the opposite side of the anchor load, the chain slack iswinched in, and when the water currents turn, the same takes place againon the opposite side. In this way, a secure and stable position isobtained as the current moves in two directions.

If production loads become extremely large, due to possible defects inthe control system so that load increases and the anchors perhaps give,this is not a catastrophe. Alternatively, booms could automatically bereleased to the surface (in the event of defects or problems with thecontrol of turbines or generators). As generator and turbine havepositive buoyancy in the water, a great amount of power is not needed toobtain this stop position.

It can also be seen that there are very substantial dynamic forces whichact on this kind of turbine, and that this in turn can result invibrations and fatigue. Compared with a fixed installation on thebottom, a floating platform will obtain considerably greater dampeningof vibrations of this type. Thus, the concept is not subject to the samedanger of fatigue.

Because the platform floats with little draught (e.g., just 1.5 meters),supporting arms 615 and turbines can easily be moved into the platformside in a “scissored position” relative to one another, and transport ortowing in the floating position will be simple.

If four propeller blades are dismantled, the whole rig can easily beplaced on most slips. Thus, underwater maintenance can be simplified (bepossible) and service life can be extended considerably.

In the next stage this results in a great second hand value, as it ismobile, but also because the overall economy of the concept may bebetter than that of a fixed anchored monopile from the bottom.

The present invention also comprises a rotating member 400 designed forrotation actuated by currents in the body of water so as to function asenergy supplier in a generator unit located in a body of water. Therotating member, as illustrated in different embodiments in FIGS. 7, 9and 10, is built up around a shaft 405 which is mounted (preferably atright angles) on a hub on the generator shaft. The rotating memberaccording to one embodiment of the invention comprises an inner endpiece 407 mounted on the shaft 405 at the attachment of the shaft to thehub, a tip 406 mounted at the outer end of the shaft, and a plurality ofmember sections 410 rotatably mounted on the shaft 405 between the endpiece 407 and the tip 406 (ref. FIG. 7).

In the embodiment shown in FIG. 7, each member section 410 isindividually selectively pivotal and adjustable about said shaft, inorder that the rotating member as such can assume the optimal pitch inrelation to the water current.

FIG. 9 shows another embodiment of the rotating member, and comprisesjust one member section 410 fixedly mounted on the shaft 405. In thisembodiment the tip 406 is selectively pivotal and adjustable about ashaft.

FIG. 10 shows yet another embodiment of the rotating member, andcomprises just one element section 410 fixedly mounted on the shaft 405.In this embodiment, the tip 406 is also fixedly mounted on the shaft.

Each member section 410 comprises at least one servo rudder 411, 412rotatably mounted at the fore and aft edge of the member section viarespective rudder axles 413. This can be seen from both FIGS. 7 and 10.The servo rudders 411, 412 are selectively and mutually independentlyrotatable by means of respective actuators 415, and can be rotated toany angle within a pre-defined range (±β₁, ±β₂) so as to assume theoptimal angle of action relative to the water current.

The plant according to the invention also comprises a generator, whichwill now be described with reference to FIGS. 11 and 12.

The fundamental idea of the invention is, on the basis of adirect-driven generator, to combine a very low input speed (substantialtorque) and a large relative speed (e.g., 25 rpm at 10 rpm of the inputshaft), by contra-rotating the stator. Thrust bearing is alsoincorporated.

FIG. 11 shows one type of the generator according to the invention. Ashaft 100, having connecting flange 102, is supported by radial andthrust bearings 110, 120. The flange 102 can be connected to a propellerif the generator is to generate electric power. A rotor 150 is fixedlyconnected to the rotor. Furthermore, a gearwheel 160 is connected to theshaft.

The gearwheel 160 engages with the driving gear bearing 335, which viathe stator driving gear 330 and the gear rim 320 causes the stator 300to rotate in the opposite direction relative to the shaft 100. FIG. 11also shows stator bearings 310, permamagnets 350 and slip rings 390. Thepermamagnets can be replaced by, e.g., electromagnets without therebydeparting from the invention.

The principle shows that the stator can be rotated in the oppositedirection of the rotor, and generally at a greater speed than the rotor.Thus, a relative rotational speed is produced that is more than twicethat at the input shaft. This makes it possible to utilise an optimalspeed of the propellers which drive the generator. This means that thestator diameter can be made relatively small in order to obtain thenecessary number of poles and frequency, and the diameter of themachinery housing is kept to a minimum in order to obtain least possibleresistance and unfavourable currents in the water. (Propellers arealternately on one side or the other of the machinery housing as thewater flow changes direction.) Since the AC voltage is in any caserectified in this type of power production in order to subsequently beconverted to alternating current, there is no need for constantfrequency. The requirement is maximum efficiency from approximately zeroto maximum speed of the propeller acting on the generator (maximumbetween 10 and 15 rpm) in connection with the fact that the tide cyclevaries.

FIG. 12 shows another type of generator according to the invention. Thegenerator comprises a rotor 550 connected to a first shaft 500 forrotation relative to a rotating stator 800 connected to a second shaft820, and with respective bearings 510, 815.

The second shaft 820 is at one end thereof connected to a stator framevia a flange 804, axially supported 810 on the first shaft 500, and thefirst shaft 500 is at one end thereof axially supported 810 in saidstator frame.

The first shaft 500 may be connected to a first propeller or turbine viaa connection 502, and the second shaft 800 may be connected to a secondpropeller or turbine via a connection 802. Slip rings 890 that are knownper se are mounted on the stator frame.

The following example illustrates the advantage of using a direct-drivengenerator having contra-rotation, compared with a conventionalgenerator, without contra-rotation, but having the same diameter andoutput.

A mechanical gear solution between the generator and propeller hasadvantages at relatively small outputs, where a standard mass-producedgenerator can be used, and where the input (driving) shaft has arelatively high speed (more than 20 to 30 rpm).

Known technology for direct drive of a generator is most appropriate atlarger outputs, and where large diameter and weight of the generatoritself are of no particular importance. If the gear weight is deducted,the total weight will still be favourable provided the input speed isnot particularly low. Although constant frequency is not required, “airgap speed” and area will determine the dimensions (provide design basis)for diameter and width (length of the generator).

If a direct-driven generator is contra-rotated at, e.g., 10 rpm of therotor and stator respectively, this will give a relative rotationalspeed in the air gap which is the sum of the speed of the rotor and thespeed of the stator, in this case 20 rpm, but the speed might easily bedifferent between rotor/stator.

Let us consider, e.g., a generator size of 350 kW. The length is set at0.75 meters and drive speed at 12.5 rpm. The required diameter iscalculated on the basis of the following formula:P=k×D ² Lnwherein: P is output (kW)

-   -   k is a constant valid for a given class of structures, in this        case set at 2.2    -   D is air gap diameter (meters)    -   L is the length of the rotor and/or stator (meters)    -   n is speed (rpm)

The air gap diameter in a standard solution as specified will be 4.12meters. The area of the magnet will be a total of 9.7 m².

If the stator and rotor are rotated in opposite directions, the relativespeed will be 25 rpm. The air gap diameter could thus be reduced to 2.91meters. At larger outputs, the differences will be even greater.

Alternatively, the gearing (the reverse gear box between rotor andstator) can be provided with a gear train. At a gearing ratio of 1:1.5,the relative speed will be 31.25 rpm.

The air gap diameter at this speed can be reduced to 2.60 meters and themagnet area can be reduced to 6.12 m².

The drawing of current from a generator with contra-rotation must beeffected via slip rings, but with the use of known solutions.

If the contra-rotation is arranged with split operation—in that twopropellers drive the generator (see FIG. 2), one the rotor and the otherthe stator, it will be possible to obtain the following:

-   -   1. Output of the generator is set in this example at 2×350        kW=700 kW.    -   2. Propeller speed of 12.5 rpm is selected—relatively this gives        25 rpm.    -   3. The length of the generator is set at 1.0 meters.

The diameter of the air gap for this solution is 3.18 meters and themagnet area is 9.78 m². That means to say the same area and costs forpermanent magnets and coils as for a standard direct-driven generator of350 kW having a diameter of 4.12 meters and a length of 0.75 meters.

Although a close study of all the details has not been made, apreliminary analysis of this type of generator operation shows areduction in the price of electricity of about 10% compared with aconcept including a “gear solution”. This assumes a 100% shadow forpropeller No. 2, which is not realistic as the distance between these is10 meters.

Furthermore, there will probably be positive output for propeller No. 2as the body of water will be made to rotate by propeller No. 1, whichrotates in the opposite direction to propeller No. 2.

The solution provides a great improvement as regards operating safetyand maintenance. Savings are estimated to be 0.5 øre per kWh/year—whichis perhaps a rather conservative estimation.

The generator according to the invention allows large torques to beutilised.

The utilisation of large torques results in optimal efficiency for boththe propeller (rotating member) and the generator. Omission of the gearresults in a saving of more than 3% loss.

Furthermore, larger generator capacity can be installed.

The risk of overloads in the event of a breakdown or failure of thecontrol system for controlling torque, thrust and output is smaller.

The arrangement results in reduced maintenance costs compared withexisting generator solutions.

Traditionally, it is the gear, or the direct-drive generator, whichtakes up the torque from the propeller. Furthermore, it is one or morethrust bearings which take up axial forces. Thus, these forces aretransmitted via the base to the machinery housing and then to the steelstructure.

For a wind turbine, this type of torque will give varying loads on thebearing of the “nacelle” and result in the slew ring bearing being anexposed element.

Because two propellers rotate in opposite directions, individualadjustment of the number of revolutions of the propellers will allow thesame torque on both shafts to be maintained, although one propellerworks in the shadow of the other.

The solution means that the torques offset each other, and that thetorque loads on the fixed support disappear. At the same time, pulsatingloads from the propellers are cushioned by the “magnet cushion” betweenthe rotor and the stator.

It should also be noted that the generator according to the inventionwill be simple to build. It can to a great extent be based onprefabricated windings and inexpensive permanent magnets.

Although the generator according to the invention in this application isdescribed as a generator for electric power, the generator can alsofunction as an electromotor, where, e.g., the shafts 500, 820 each drivea contra-rotating propeller. Such units can be used in air and in water,and may be particularly suitable in so-called pod thrusters in ships andother vessels.

List of Components Shown in the Figures

chgd. orig. 600 control room 610 float member 615 supporting arms 616joints 617 mooring element 620 supporting structure 700 100 nodalelement 710 110 strut 711 111 upper bushing 712 112 lower bushing 720120 horizontal supporting member 730 130 connecting member 740 200intermediate connecting member 749 300 vertical supporting member 750350 generator unit 752 352 generator housing 400 rotating member 405shaft 406 tip 407 inner end facing hub 410 member section 411 forwardservo rudder 412 rear servo rudder 413 rudder axle 414 seal betweenmember section and servo rudder 415 actuator B 500 bottom below thewater S 550 surface of the water 760 600 supporting legs 100 shaft 102connecting flange 110 shaft bearing (is secured) 120 shaft bearing 150rotor 160 gearwheel 300 stator 310 stator bearing 320 stator gear rim330 stator driving gear 335 driving gear bearing 350 permamagnets 390slip rings 500 rotor shaft 502 connecting flange 510 shaft bearing (issecured) 550 rotor 800 stator 802 connecting flange 804 flange 810stator bearing 815 shaft bearing (is secured) 820 stator shaft 890 sliprings

1. A floating plant for the production of power from currents in a bodyof water, comprising a supporting structure supported by at least onepair of float members which extend substantially laterally from saidsupport structure, wherein the float members in each pair are arrangedon opposite sides of the supporting structure, and that a plurality ofsupporting arms for carrying power generators extend laterally on eachside of the supporting structure and generally transversely with respectto the supporting structure's longitudinal direction, each one of saidsupporting arms being pivotally attached to the supporting structure atone end and supporting a power generator along a second end.
 2. A plantaccording to claim 1, characterised in that the supporting arms compriseat least one joint between their first and second ends, so that eachsupporting arm can be folded about the joint, in order thereby,concurrently with the swinging of the supporting arm about the pivotalconnection of its first end with the structure, to raise the respectivegenerator above the surface of the water.
 3. A plant according to claim1, characterised in that an individual generator unit is pivotallysupported about a vertical axis at the second end of the respectivesupporting arm.
 4. A plant according to claim 1, characterised in thatan individual generator unit is fixedly mounted at the second end of thesupporting arm.
 5. A plant according to claim 1, characterised in thateach generator unit includes a shaft which is equipped with at least onerotating member.
 6. A plant according to claim 1, characterised in thateach generator unit includes a shaft which is equipped with at least onerotating member rotationally mounted on each side of said generator. 7.A plant according to claim 5, characterised in that the rotating membersare counter-rotating.
 8. A plant according to claim 6, characterized inthat the rotating members are counter-rotating.
 9. A floating plant forthe production of power from currents in a body of water, comprising: asupporting structure supported by at least one pair of float membersextending from the support structure; a plurality of hinged supportingarms for carrying power generators which extend laterally on each sideof the supporting structure and generally transversely with respect tothe supporting structure's longitudinal direction, each one of saidsupporting arms being pivotally attached to the supporting structure atone end and supporting a power generator along a second end; and ahydraulic drive mechanism for folding said support arms to selectivelyraise or lower said generators.
 10. A plant according to claim 9,characterised in that an individual generator unit is pivotallysupported about a vertical axis at the second end of the respectivesupport arm.
 11. A plant according to claim 10, characterised in thateach generator unit includes a shaft is equipped with at least onerotating member.
 12. A plant according to claim 10, characterised inthat each generator unit includes a shaft which is equipped with atleast one rotating member rotationally mounted on each side of saidgenerator.
 13. A plant according to claim 11, characterised in that therotating members are counter-rotating.
 14. A plant according to claim12, characterized in that the rotating members are counter-rotating.